A differential is a device that allows the driven wheels of a vehicle to turn at different speeds such as when making a turn. In rear wheel driven cars, the differential is known as the Rear End. There are essentially two types of differentials which can be described as heavy and light duty. The heavy duty, used by the automotive industry, is designed for heavy loads, high speeds and 1000's of hours of life. The light duty, used by the lawn and garden industry, is designed for light loads, low speeds and 100's of hours of life. Consequently, the designs of the two types are vastly different. The former requires accurate, well-supported steel gears, while the latter have simply, often highly unconventionally, supported powdered metal gears.
Generally, the differentials of the type used in lawn and garden tractors have the following components: a ring gear, two axle shafts, two planet gears, two bevel gears, a cross-shaft and various thrust washers and retaining rings. These parts are contained in and supported by a first housing portion and a second housing portion that may or may not have separate bearings.
The first and second housing portions mate to form a housing for the transmission. The differential is a part of the transmission. It should be noted that most of the time, the terminology for the components is as used above. Sometimes, however, the bevel gears are referred to as sun gears; the planet gears are referred to as planet pinions or merely pinion gears; and the ring gear is called the crown wheel.
Two types of transmissions are used in lawn and garden tractors, namely the mechanical and the more easily operated hydrostatic transmission (HST). Because of the high cost of the Hydrostatic transmission, most of the transmissions used in the past have been mechanical.
Over the last few years, however, the cost of the hydrostatic transmission has been substantially reduced. This lower cost and the ease of operation has resulted in the hydrostatic transimission gaining an increasingly greater share of the market. However, the hydrostatic transimission is less efficient than a mechanical transmission. To compensate for the inefficiencies, larger engines are required. The larger engine and inefficiencies translate into heat. Therefore, special features must be added to cool the hydrostatic transimission. In other words, the housing must be more complex with additional fins and added surface area to carry away the added heat generated. In addition, cooling fans must be added to force ambient air around the housing to adequately cool the unit.
Common types of differentials used in mechanical transmissions are very inefficient because the ring gear is supported on the Major Diameter (M.D.), or on cylindrical surfaces at a large diameter, by a cavity in the housing which may or may not have an intermediate member of steel. In any case, the friction is high and the diameter large. As Horsepower (HP) is proportional to Torque (T) and Speed (N), and as T is proportional to the gear Force (F), Radius (R) to the supporting surface, and Friction (u), Horsepower Loss (HPL) is proportional to N, F, R, and u. For purposes of comparison N and F are constants and consequently HPL is proportional to R and u, which, in the case of the more commonly used differentials are both very large. Additionally, as the ring gear floats in the housing cavity, it continuously bumps up against the housing at a large diameter further increasing the HPL.
As the hydrostatic transmission is a rather inefficient device, any inefficiencies downstream of the hydrostatic transmission further aggravates the problem of cooling the hydrostatic transmission because the hydraulic units of the hydrostatic transmission must work that much harder. Consequently, hydrostatic transmissions as described in U.S. Pat. Nos. 4,339,965 and 5,447,028 have tried to address the problem of high differential inefficiencies by supporting the ring gear by a highly finished shaft of small diameter.
The construction of the hydrostatic transmission of U.S. Pat. No. 4,339,965 still allows the gear to float laterally. The cross-shaft is also expensive to produce. Additionally, the necessary clearances between the cross-shaft and the ring gear will, over time, cause wear and shortened life of the Hydrostatic transmission.
The construction of the hydrostatic transmission of U.S. Pat. No. 5,447,028 overcomes some of the problems of U.S. Pat. No. 4,339,965 mentioned above, but at increased cost. More parts and special assembly techniques are probably required to assure proper gear alignment of this hydrostatic transmission.
Therefore, there is a need for a low cost more efficient differential that can be used as part of either a mechanical or a hydrostatic transmission. A more efficient differential will have less wear and will last longer while not producing heat as a result of friction between the various components and the housing of the transmission. In addition, there is also a need for a transmission that has fewer parts and is simple to assemble and manufacture. There is a further need for a transmission that can be made less inexpensively.